This invention relates generally to a process for removing low levels of ethylene oxide from ethylene oxide contaminated gases using ion exchange resins at gas-solid interface reaction conditions providing rapid reaction rates and bound polymer reaction products, which are retained on the resins.
Removal of ethylene oxide from air is industrially important to maintain environmental quality and to protect workers' health. Various designs for scrubbers are commercially available, all working on the same principle: The gas mixture, containing traces of ethylene oxide, is contacted with water. The water contains a dissolved catalyst, typically a strong acid such as sulfuric acid, or a strong base. This catalyst promotes the reaction of ethylene oxide with water (hydrolysis) to form ethylene glycol, which is a relatively innocuous and non-volatile compound. This scrubbing liquor is reused until the process efficiency decreases beyond an acceptable limit, due to the accumulation of ethylene glycol; at this point, the pH of the liquor must be adjusted (neutralized) before disposal.
While acid scrubbers perform adequately for process streams with moderate levels of ethylene oxide in air, reduction of ethylene oxide concentrations below a few parts-per-million is generally impractical. Likewise, room air cleaning, which involves larger air volumes and lower concentrations of ethylene oxide, cannot be performed with wet scrubbing unless prohibitively large water flow rates and huge equipment are employed.
The other systems currently available for ethylene oxide removal are based upon adsorption on activated carbon. Activated carbons have reasonable capacity for ethylene oxide sorption at higher gas-phase concentrations, but below about 100 ppm their capacities are too low to be considered. Moreover, because ethylene oxide is retained on activated charcoal by comparatively weak retention forces of absorbtion, description may occur and the effectiveness of removal can vary unacceptably over time and under various conditions.
Ethylene oxide contaminant removal, at high levels, such as found in sterilizer effluents using exchange resins, is taught in U.S. Pat. No. 3,841,053, which discloses a system and method for the removal of ethylene oxide contaminants using gel-type ion exchange resins. In accordance with the method of U.S. Pat. No. 3,841,053, the presence of water is required. Water is present in such an amount that ethylene oxide is reacted with the resin at a liquid solid interface; that is the reaction is effected in a liquid phase.
Ion exchange resins have also been used in methods of 1,4-Dioxane synthesis (See e.g. U.S. Pat. No. 4,365,071 and German Pat. No. 2,430,355) wherein an ethylene oxide stream is reacted with the resin to form an intermediate product. In such processes, once again, the intermediate reaction involving the resin and ethylene oxide is effected in liquid phase.
Specifically, acidic sulphonated polystyrene resins have been used in the prior art to catalyze the formation of a liquid ethylene oxide product (ethylene glycol and polyglycol liquids) in the presence of water in order to quantitatively measure ethylene oxide exposure. (U.S. Pat. No. 4,423,005)
The art also teaches the use of ion exchange resins as catalysts in the high temperature gas-phase reactions of ethylene-oxide to form ethylene glycol.
The present invention is based on the discovery that low level ethylene oxide contaminants can be removed from a contaminated gas by reaction of the ethylene oxide with dried acidic ion exchange resins, by using, contrary to the prior art (U.S. Pat. No. 3,841,053)--a gaseous rather than liquid interface with the resin. By use of a gas-solid reaction interface, bound ethylene oxide polymer is rapidly formed and retained on the resin at the conditions of ethylene oxide removal.
In contrast to U.S. Pat. No. 4,423,005, this invention enlists ion exchange resins in systems where ethylene oxide and water concentrations are much more dilute than those encountered in sterilizer operations. In this regime, reaction products are formed which are largely bound to the resin, and water behaves as a rate inhibitor.
Much of the prior art described above depends upon ion exchange resins to catalyze reactions of ethylene oxide. In the present invention, ion exchange resins are employed, to a large extent, as solid phase reactants.